Lithium ion secondary batteries are being widely used as a power source for portable compact electronic devices such as, for example, cell phones, personal digital assistants (PDAs), notebook computers and camcorders, since they have a high capacity and a high energy density and facilitate miniaturization and weight reduction. With the recent trend toward increasing the functionality of portable compact electronic devices, demand is also growing for higher capacity in lithium ion secondary batteries.
Various methods have been proposed to achieve higher capacity in lithium ion secondary batteries. Among them, attention is focused on an alloy-based negative electrode active material. The alloy-based negative electrode active material is a material capable of absorbing lithium by being alloyed with lithium and reversibly absorbing and desorbing lithium. As the alloy-based negative electrode active material, for example, silicon, silicon-containing compounds, tin, tin-containing compounds and the like are known. Because the alloy-based negative electrode active material has a high discharge capacity, the use of such a negative electrode active material is effective in achieving higher capacity in lithium ion secondary batteries. Silicon, for example, has a theoretical discharge capacity of about 4199 mAh/g, which is about 11 times the theoretical discharge capacity of graphite.
However, an alloy-based negative electrode active material expands because the crystal structure changes significantly when absorbing lithium ions, causing not only the current collector but also the negative electrode to deform. Along with this deformation, cracking of the negative electrode active material particles, separation of the negative electrode active material layer from the current collector, and the like occur. As a result, the electron conductivity between the current collector and the negative electrode active material layer decreases, leading to the deterioration of battery characteristics such as cycle characteristics. In order to solve such problems, a proposal has been made in which a space for allowing such expansion when absorbing lithium ions to occur is provided in advance in such an alloy-based negative electrode active material layer.
Specifically, a negative electrode for a lithium secondary battery has been proposed in which, for example, a thin-film negative electrode active material layer made of an alloy-based negative electrode active material or an alloy containing an alloy-based negative electrode active material is formed in a prescribed pattern on a current collector made of a material that does not alloy with lithium (see, for example, Patent Document 1). According to Patent Document 1, such a thin-film negative electrode active material layer is composed of a plurality of columns, and the columns are arranged in a staggered arrangement, a grid arrangement or the like with a gap between adjacent columns. And, by causing the gap between columns to absorb the volume expansion of the alloy-based negative electrode active material contained in the columns, deformation of the negative electrode current collector, as well as cracking of the alloy-based negative electrode active material, separation of the columns from the current collector, and the like that occur along with such deformation are prevented.
Another negative electrode for a lithium ion secondary battery has also been proposed which includes a current collector having a surface roughness Ra of 0.01 μm or more, and a thin-film negative electrode active material layer formed on the surface of the current collector and containing an alloy-based negative electrode active material (see for example, Patent Document 2). By providing a negative electrode of the above configuration, Patent Document 2 attempts to increase the contact area of the interface between the current collector and the thin-film negative electrode active material layer and improve adhesion between the current collector and the thin-film negative electrode active material layer.
Furthermore, when the negative electrode of Patent Document 2 is included in a lithium ion secondary battery, and the battery is subjected to an initial charge and discharge, because the current collector having the above-mentioned surface roughness is used, cracks are formed in the thin-film negative electrode active material layer, and the thin-film negative electrode active material layer is divided into a plurality of columns which exist apart from each other with a gap therebetween. Because the gap absorbs the expansion of the alloy-based negative electrode active material, separation of the thin-film negative electrode active material layer and the like are further prevented.
Attempts have also been made to reduce the expansion and contraction of the alloy-based negative electrode active material. For example, a proposal is disclosed in which a partially nitrided silicon oxide powder represented by a composition formula: SiNxOy (where, 0<x<1.3, 0<y<1.5) is used as a negative electrode active material for a lithium ion secondary battery (see, for example, Patent Document 3). By introducing nitrogen into silicon oxide, the degree of expansion and contraction is reliably reduced although the discharge capacity decreases slightly. In an example of Patent Document 3, a negative electrode is produced in which a negative electrode active material layer containing the partially nitrided silicon oxide powder is formed on the current collector surface.
A technique has also been proposed in which lithium foil is firmly attached to the surface of the negative electrode active material layer of a negative electrode with an auxiliary layer containing water-insoluble conductive particles interposed therebetween (see for example, Patent Document 4). The lithium in the lithium foil firmly attached to the negative electrode active material layer starts to disperse into the negative electrode active material layer at the point in time when an electrolyte is injected, and disperses throughout the negative electrode active material layer during the initial charge and discharge. Patent Document 4 also discloses a negative electrode active material layer configured to have a opposing portion opposing a positive electrode and a non-opposing portion not opposing the positive electrode, wherein the amount of lithium per unit area firmly attached to the non-opposing portion is increased relative to the amount of lithium per unit area firmly attached to the opposing portion.
Conventionally, a method is used in which a lithium film is formed on the surface of a negative electrode active material layer by a vapor deposition method, such as a vacuum deposition method or ion plating method, so as to cause the negative electrode active material layer to absorb lithium that does not take part in charge and discharge reactions in an amount equivalent to its irreversible capacity. Another method is also known in which a lithium film is formed, by a vapor deposition method, on the surface of a negative electrode active material layer formed by a vapor deposition method (see for example, Patent Document 5, paragraph 0037; and Patent Document 6). However, according to the conventional methods, the lithium film is formed only in the opposing portion of the negative electrode active material layer, and not on the non-opposing portion. Patent Documents 5 and 6 do not disclose forming a lithium film on the non-opposing portion.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2004-127561
Patent Document 2: WO 01/031722 Pamphlet
Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-356314
Patent Document 4: Japanese Laid-Open Patent Publication No. Hei 9-283179
Patent Document 5: Japanese Laid-Open Patent Publication No. 2005-85632
Patent Document 6: Japanese Laid-Open Patent Publication No. 2005-38720